1. Field of the Invention
The present invention relates to wire-wound common mode choke coils for eliminating common mode noises on transmission paths.
2. Description of the Related Art
Conventional common mode choke coils of this type are disclosed in Japanese Unexamined Patent Application Publication No. 2003-168611 and Japanese Unexamined Patent Application Publication No. 2000-133522.
A common mode choke coil of this type has a configuration in which two wires are wound around a winding core potion of a core having flanges on respective sides, ends of each of the wires are connected to electrodes provided at the flanges on the respective sides, and a ferrite plate is arranged on an upper surface side of the flanges.
With this configuration, it is possible to eliminate common mode noises in differential transmission paths of a CAN (Controller Area Network) or the like.
However, the above-described conventional common mode choke coils have a problem as described below.
In general, an immunity test is performed before products are put on the market to examine whether the products can tolerate various kinds of electromagnetic interference by exposing the products to expected electromagnetic interference.
In the immunity test of a common mode choke coil against common mode noises, a common mode choke coil defining a test-target product is arranged upstream of a reception IC, which is connected to a transmission IC through differential transmission paths. Differential signals are transmitted from the transmission IC to the reception IC through the differential transmission paths and common mode noises of, for example, 1 MHz to 400 MHz are generated on the differential transmission paths, whereby these common mode noises are superposed on the differential signals. In such a state, the transmission IC or the reception IC malfunctions is checked to determine whether it malfunctions.
However, since the inductance of the common mode choke coil defining the test-target product and the input capacitance of the reception IC define a resonant circuit at the time of such an immunity test, a ratio of suppressing common mode noises drops at a resonant frequency of this resonant circuit and a frequency band near the resonant frequency. In such a case, the transmission IC or the reception IC malfunctions and a problem in that the test-target product does not pass the immunity test occurs.